Obesity in C57BL/6J mice fed diets differing in carbohydrate and fat but not energy content.

To investigate the contributions of carbohydrate and fat to obesity we measured the body weight, body composition and food intake of adult C57BL/6J mice fed ad libitum with various combinations of two semisynthetic diets that differed in carbohydrate and fat but not in protein, micronutrient or energy content. In Experiment 1, involving male mice, body weights were similar in groups fed diets comprised of (by energy) 20% protein, 75% carbohydrate and 5% fat (C75-F5) or 20% protein, 5% carbohydrate and 75% fat (C5-F75). However, mice fed a 50:50 composite mixture of the C75-F5 and C5-F75 diets (i.e., a C40-F40 diet) became substantially more obese. Mice that could choose between the C75-F5 and C5-F75 diets ate equal amounts of each diet and gained almost as much weight as did the group fed C40-F40 diet. Mice switched every day between the C75-F5 and C5-F75 diets gained no more weight than did those fed either diet exclusively. In Experiment 2, male and female mice were fed chow or one of 8 isocaloric diets that differed parametrically in carbohydrate and fat content. Groups fed diets in the middle of the range (i.e., C35-F45 or C45-F35) weighed significantly more and were significantly fatter than were those fed diets with more extreme proportions of carbohydrate and fat (e.g., C75-F5, C5-F75), an effect that was more pronounced in males than females. In Experiment 3 and 4, male mice fed versions of the C40-F40 formulation gained more weight than did those fed the C75-F5 or C5-F75 formulations irrespective of whether the carbohydrate was predominantly sucrose or predominantly starch, or whether the fat was vegetable shortening, corn oil, palm oil or canola oil; the type of carbohydrate or fat had little or no impact on body weight. In all four experiments, energy intakes differed among the diet groups but could not account for the differences in body weight. These results demonstrate that the proportion of carbohydrate and fat in the diet influences body weight independently of energy content, and that the type of carbohydrate or fat has little impact on body weight. Consuming carbohydrate and fat simultaneously or in close temporal proximity exacerbates obesity.